This invention relates to aqueous compositions, e.g. coatings such as latex paints and to the process of preparing aqueous dispersions of pigment and a polymeric resin in an aqueous medium with a dispersing agent consisting of half-esters of starch characterized as having pendant carboxyl groups and an average degree of substitution ranging from 0.25 to 3.0 wherein at least 10 mole percent of said pendant carboxyl acid groups are neutralized. These half-esters of starch or dispersants are derived from low molecular weight hyrolyzed starch molecules having a plurality of anhydroglucose units and acylating agents consisting of at least one cyclic anhydride of polycarboxylic acids.
The incentive for developing dispersants derived from starches is the uncertainty over the availability of similar materials derived from petrochemicals. A way to avoid the problems encountered because of shortages of compounds prepared from petrochemicals is to diversify the raw materials from which these compounds are derived by relying on agricultural products. These products are renewed on an annual basis and the volume, if necessary, can be expanded in accordance with the demands by increasing the acreage. For example, starch which is derived from corn, potatoes and the like is presently being produced at rates exceeding 10 billion pounds per year and is being used for a variety of industrial applications in its natural form. However, starch in most instances must be chemically modified, e.g. esterified, etc. before products are obtained with the required characteristics and particularly for use in aqueous systems. Presently, low molecular weight starches with high degrees of substitution are not commercially available and for the most part only the very high molecular weight starches with low degrees of substitution are being used in most commercial applications.
The standard grade of high molecular weight starches consist of approximately 80% amylopectin with average molecular weights ranging from 1 to 30 million and about 20 percent of amylose with average molecular weights ranging from about 40,000 to 300,000. The structure of amylopectin and amylose may be illustrated by Formulas I and II wherein n is the number of linear and branched anhydrolgucose repeating units. ##STR1##
In comparison to these starches which comprise high molecular weight polymeric chains of amylose and amylopectin, the hydrolyzed starches of this invention used in preparing the dispersants comprise depolymerized chains of amylose and amylopectin with average molecular weights ranging only up to about 100,000 and more likely ranging up to about 80,000, e.g. from about 400 to 80,000. The comparatively low molecular weight starches are obtained from the high molecular weight materials by known techniques. These starches are specifically illustrated in Table I wherein the average molecular weight was determined by gel permeation chromatography with polydextran standards and filtered deionized water as the carrier solvent. In addition to the materials recited in the Table, there are numerous other low molecular weight starches and various derivatives thereof with low degrees of substitution which can be used for purposes of this invention.
TABLE I __________________________________________________________________________ Starch Approx. Average Materials Type D.E. Supplier Molecular Weight __________________________________________________________________________ Fro-Dex 42 Dried corn syrup 12 American Maize-Products Co. 600 Fro-Dex 15 Maltodextrin 18 American Maize-Products Co. 16,000 Amaizo 1736 Dextrin 4 American Maize-Products Co. 16,000 Mor-Rex 1918 Cereal Solids 10 CPC International 18,000 Experimental Low D. E. Sample Hydrolysate 5 American Maize-Products Co. 35,000 RL 5540 Enzyme Hydrolysate 0 SW Laboratories 86,000 __________________________________________________________________________
While much attention has been given to the higher molecular weight starches, comparatively little has been done with the low molecular weight materials especially the low molecular weight starches with high degrees of substitution. There are many advantages in using lower molecular weight starches and particularly important is the ability to control solubility which provides a means of preparing highly concentrated solutions of workable viscosity.
The hydrolyzed, low molecular weight starches may be characterized as comprising branched and linear anhydroglucose units, but more likely consist essentially of linear or straight polymeric chains since many of the branched points were cleaved during hydrolysis.
It has been found that the low molecular weight starches which have been highly substituted by esterification can be used effectively as dispersants in aqueous systems and particularly in aqueous coating systems. The properties of the hydrolyzed starch molecules can be varied or controlled largely by the extent and type of acylating agent, i.e. by the cyclic anhydrides. Starch, for example, is substantially a polymer of glucose having three hydroxyl groups for each repeating unit (see the structure of amylose and amylopectin) and by converting all or substantially all of the hydroxyl groups of each anhydroglucose unit to ester groups, the esterified starch may then be characterized as having a degree of substitution or D. S. value of 3.0. This is the maximum degree of substitution but lower degrees of substitution can be obtained by controlling the relative amounts of the reactants, i.e. the hydrolyzed starch and the anhydride in esterification. For purposes of this invention, the degree of substitution is essential and, therefore, must fall within the range of about 0.25 up to 3.0 and preferably should be greater than 0.5. The D.S. value is therefore a means of characterizing the number of substituents, e.g. ester groups per anhydroglucose unit.
It is obvious that various other derivatives of the low molecular weight hydrolyzed starches characterized as having low degrees of substitution, i.e. below about 0.1 can be used also either alone or in combination in any proportion with the unsubstituted hydrolyzed starches in preparing the esterified products.
Accordingly, it is an object of this invention to provide comparatively low molecular weight half-esters of starch as dispersants for latex coating compositions. It is another object of this invention to provide esterified low molecular weight half-esters of starch having a high degree of substitution as dispersants for pigments in aqueous systems. It is another object of this invention to provide low molecular weight anionic half-esters of starch having high D. S. values and pendant carboxyl groups with various hydrophilic/hydrophobic characteristics as dispersants in latex coatings. It is further an object of this invention to provide an aqueous coating composition comprising half-esters of starch as a dispersant to improve the hiding efficiency of the pigment.